Okay, thinking out loud:

so what would he have to hold for you to lose this:
Js, Ts, 9s, 7s

Anything other than that is a split, and it costs you E3.5 to find out which way it's going to go.

He called a 7.5xbb preflop raise (total) after making a 3xbb ish raise himself.

So if he was playing JJ,TT,99 that would make sense. What other hands would call your preflop: AK, KK,QQ also? All of those would split with you. So maybe a 50/50 chance that you're going to split it.

Irritatingly, there's only E0.3 in there that's dead money.

I think I'd be cursing and calling and hoping he's got AK, guessing that 50% of the time he has the spade balances the 50% of the time that you get your money back.