I have a question relating to correct use of the word 'spend'. Perhaps you can clarify whether a), b), or both are grammatically correct, and why?
a) With every $30 spend in a single receipt, you will receive a free gift.
b) With every $30 spent in a single receipt, you will receive a free gift.
The statement is for use in some marketing literature, to promote sales. You inputs will help to resolve an office conflict!