Personally in your situation would ignore the request for cash.

Then I would scour the second hand places (we call the op shops over here) and get them a present. Just because it is from an op shop it doesn't have to look used.

It truly is amazing what good things people recycle simple because it just isnt their style or maybe an unwanted gift. Quite a few new things land in there and are very nice. Maybe you could pick up a piggy bank and joke that they can put there money from the wedding in it?