Credit Crunch times Milli - I wouldn't hesitate to use it.
Also, I would expect (as a chivalrous man) him to offer to pay anyway and be grateful for the voucher!
So...
If he expects you to pay, use the voucher and be a bit aggrieved that he isn't offering to pay
If he is offering to pay, you can politely off the voucher which he should appreciate
Or, if he is expecting to split it and pay individually, take his cash and send him to the ice cream van while you buy the tickets using the voucher for your half.
Maybe I am too old fashioned...