Offer him the price he proposed (which you have already done) with a "take it or leave it" attitude? How much did he propose? Maybe just state that since he's proposed a price you'll only offer that or lower.

Maybe offer a small share of what you're setting up? There's all sorts of legal and organisational issues with that though.
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Christian
#40104192 120Gb (no longer in my E36 M3, won't fit the E46 M3)