Yep, except the "scratch" one has a slightly negative connotation of doing something just because the other one will do something in return. The Spanish phrase is a bit more positive.
Actually, the best part about the "you scratch my back and I'll scratch yours" is that it refers to whipping. It's an expression that comes from ship life, where shipmates would be tasked with whipping each other when they misbehaved. The idea was that if you whip me more softly (scratch my back), then when it's my turn to whip you I'll do the same.
I notice this explanation seems to be commonly favoured by google, but it is unlikely.
Floggings in the Royal Navy were done by the bosun's mate, not a random crew member. The bosun and the lieutenants would watch carefully for favouritism. There was no rancour directed towards the mate because everyone knew he was just doing his job.
I agree, since there is no implication in the Spanish phrase of "what can you offer in return?". The Golden Rule seems to mesh better with the original ideal expressed. I always find it interesting when literal translation, or convenience in closeness of expressions, are preferred over intended meaning.
