Money is such a petty reason...I am by no means a "wealthy" college student, and nor are my friends. But there is an understanding that sometimes I buy the beers or drive real far and burn my diesel or I am the DD or I repair my friends car for free, and sometimes they do the same for me. If one hand washes the other, money is never a problem. I personally hate "friends" who are always asking you about money for owing them "this or that" when you do so many things for them.
With my close friends, money could never be a problem. More trust or ditching to hang out with the girlfriend and not being man enough to say or simply just taking advantage of friends. One hand washes the other with most of my friends, but with a few who don't have so much scratch I've been washing a lot of hands.
In the long run, if he's a good friend, he'll get you back. Thats how I look at it. And if he doesn't, doing something nice for someone never hurts.

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1976 300D
190,000 Miles
Colorado Beige

1975 300D
Parts Car
78,000 Miles
Rustbucket
Also Colorado Beige

1984 190D 2.2 (Dad's)
156,000 miles
Champagne Metallic Clearcoat