to put it short I was out with "that model" all day til midnight, then drove to where my offshore friends are and they were very drunk, catching up with them etc, showed one my nazi helmet and he borrowed it sieg heiling down the street cop car even stopped to take a look at him then drove off again, gatecrashed into this weird army party, acquantance girl I kinda know she asked for a lift to the fuel station to buy some cigarettes so I drive there, I put £10 of diesel in and acquantance girl is at the desk paying for her cigarettes, I only have my credit card to pay for the fuel and the fuel lady says that girl already paid for your fuel and I was like "wtf" and I feel bad now and I said to her "next time I see you I will be giving you that £10 back but she insists on not wanting to be paid back so I said something about shouldnt do that as people will make a mug of you etc and I didnt want to do that to just let her pay out of the blue, fuel station trip was only a 2 min drive so its appropiate to just let it drop or actually pay her back? but dunno if it would appear rude or ungrateful if I do try to force give back the money

any suggestions?

Can't you just say 'Thank You' to her for such a kind gesture? People do nice things occasionally and you'll probably get the chance to make a reciprocal gesture in the future.

Man, I'd hate it if everyone gave me money for petrol when I give them a lift.
(They never do, I never accept it, but it's always graciously received.)

Say thank you.
Then see a priest about exorcising that helmet - giving you weird dreams, making your mate goose step. It's clearly possessed.

Graciously accept the money and send some of your awesomely random acquaintances to Lanzarote, I could do with a different kind of tourist for a while.

Accept it with a thank you and buy her a surprise present with it.

Buy her drink next time you see her.