The most graceful way to deal with the situation is by asking an indirect question. Make sure to use your most neutral tone of voice when using the following examples.
- Were you planning on paying me today or next time?
- I can't remember, did you pay for this week last week?
- Will you need cash back today?
- Will you want me to send you an invoice?
- Did you need my address to send me the check?
Picture from http://www.nightingalebullies.com/purchase.html