There's no prohibition about spending money on a Sunday.
Some theologians such as Fr. Heribert Jone (
Moral Theology, TAN,) would seem to very likely place a prohibition on unnecessarily going to a restaurant on Sunday. On p.122 of said book he states: "Forbidden in themselves are such activities as marketing, fairs,
buying and selling, public auctions, shopping in stores."
The 1917 Catholic Encyclopedia under the entry for Sunday reads as follows: " From the eight century the
law began to be formulated as it exists at the present day, and the local councils forbade servile work, public
buying and selling,....."
To go out and buy a meal on Sunday when it is not necessary may well be in keeping with the letter of the law, but it would seem to be of some real doubt as to whether it was in keeping with the spirit of the law, i.e. the Third Commandment.I am of the impression that although most Trads would agree that we should try to refrain from unnecessary buying at department stores on Sunday many of them find nothing wrong in buying tickets to entertainment events or to buying a meal on Sunday when it is not necessary. It seems to me that by doing the latter we feed into the secular culture (which profanes Sundays in every way imaginable) by treating Sundays like any other day of the week.
It may be of some of some interest that Sr. Maie of St. Peter, O.C.D., the nun and visionary mystic who is best known for starting the devotion to the Holy Face of Jesus and for the Golden Arrow Prayer, asserted that in one of her private revelations concerning reparation to Christ's Holy Face, Jesus told her: "
The executioners crucified me on Friday, Christians crucify me on Sunday." Interestingly enough this revelation was given the following year (1847) after Our Lady of LaSallete wept over man's neglect of Sunday