[Avodah] shaveh prutah

[Danny Schoemann]

R' David Riceman <driceman at optimum.net> asked: Suppose a store gives
away such copious amounts of coupons that there is no secondary
market, the coupons say on their face that they're worth one tenth of
a prutah, but that they entitle the bearer to a discount of 3 prutos
on every dinar of purchases.  Is the coupon shaveh prutah? How do you
know?

How is a coupon different from any other Shtar (legal document) that
has no intrinsic monetary value?

- Danny