The use of the word against here means in exchange for (vouchers redeemable against future purchases) here but its origin is from accounting.
In accounting (specifically, double-entry bookkeeping), there are two columns of figures.
Any item in Column A will have a corresponding item in Column B. One item is said to be recorded against another.
Here is an example of this: credit/debit
So, the word against means on the same line in another column of an account.
The term's origin is that and its meaning is: in exchange for. The value of the vouchers are entered (recorded) in one column and the value of future purchases are entered (recorded) on the same line in another i.e. against them.
In everyday language, x redeemable against y comes from bookkeeping.