I have read several references to the medieval French practice of
marriage "by solidus and denarius" ("per solidum et denarium").  This
was apprently the practice of the groom giving the bride a solidus and
a denarius (a gold coin and a silver coin) either at the betrothal or
at the wedding.

Although I have seen brief mention of this ceremony in several places, the
only "primary source" I've been able to find is in the Chronicle of
Fredegar (book III, 18,19) where Clovis' envoy to Chlothild presents her with 
"a solidus and a denarius, as was the custom of the Franks."  Neither
this nor any other reference I've found gives any details about this
"custom of the Franks."

I'm looking for another primary source for this custom.  I think this
will be a tough one, so I'm willing to be a little flexible in what I
accept, but I want more than just a vague statment that "this was the
custom", and I don't want a reference that just repeats Fredegar.

The custom appears to derive from Salic Law:

The Salic Law (Lex Salica) is a code of law written around the time of
Clovis (476-96) for the Salian Franks, in Latin mixed with Germanic
words. It deals mainly with monetary compensations (wehrgeld) and also
with civil law with respect to men and land....The Salic Law was
reformulated under Charlemagne and still applied in the 9th century,
but it slowly disappeared as it became incorporated into local common
laws. By the 14th century it was completely forgotten.

Here is an interesting link:

http://users.pullman.com/fjstevens/tokens/ArrasTokens/FrenchArras.htm

I hope this is of some assistance to you,

Josh

Thanks for the comment!  I've looked into Salic Law a bit, and there
are lots of fines for breaking various rules related to marriage, but
I haven't found anything that directly relates to my questions.

If someone can point me to a place in Salic Law where it specifies
payment of a solidius and a denarius for entering into a "regular"
marriage, I would count that as an answer!

(A "regular" marriage as opposed to some special case, such as a widow remarrying.)

According to the article "Ritual of Marriage" as published in "The
Catholic Encyclopedia", Volume IX, 1910:

"Most remarkable of all perhaps is the giving of gold and silver by
the bridegroom to the bride."
"This action takes us back to Tacitus's account of German marriage
customs. "The wife", he says, "does not present a dower to her
husband, but the husband to the wife" (Germania, xviii). Undoubtedly
this is a trace of the primitive sale by which the bridegroom paid a
sum of money for the transference to him of the "mund" or right of
custody of the bride. Originally that money was paid to the father or
guardian, but by successive stages it became a sort of dower for the
bride and was represented by the symbolical payment to her of "arrhæ
", the name by which the money thus given in the marriage ceremony is
still designated. In certain branches of the Teutonic family, notably
the Salians, this form of purchase of a bride was known as marriage
"per solidum et denarium"."
http://www.newadvent.org/cathen/09703b.htm

The Catholic Encyclopedia:
http://www.newadvent.org/cathen/

Tacitus, Germania:

http://www.gutenberg.net/dirs/etext01/tctgr10.txt
http://onlinebooks.library.upenn.edu/webbin/gutbook/lookup?num=2995

http://www.unrv.com/tacitus/tacitus-germania-4.php

I suppose Tacitus could be regared as another primary source for this
custom. However, I may be mistaken as the article mentioned was
written nearly a century ago and recent research may have led to other
conclusions.

Thanks for the comment about Tacitus!  Unfortunately, his description
is so general that it is impossible to tell if it is related to
"marriage by solidius and denarius."  The custom of giving gifts at
weddings is nearly universal, and while the "direction" Tacitus
mentions (husband giving to wife) is the same as the Arras,, that's
not enough to make a secure connection.

This book could be helpful. However, it seems to be out of print.

The laws of the Salian Franks
translated and with an introduction by Katherine Fischer Drew
Publisher: Philadelphia : University of Pennsylvania Press, 1991.
ISBN: 0-81228-256-6

http://isbndb.com/d/book/the_laws_of_the_salian_franks.html

With the help of this webpage you should be able to find a library
having a copy of this book (however, I am not quite sure if this link
will work):
http://www.worldcatlibraries.org/wcpa/ow/2c5a2e523e558b8ba19afeb4da09e526.html

ISBN of the paperback edition: 081221322X
http://home.achilles.net/~flareau/codification-Francs.html

Thanks!  I've seen the Drew book, and while it has a lot of good
information, it doesn't mention "marriage by solidus and denarius."