Is it SPAM?
No. Not by the network's standards. See the FAQ What are the “spam” and “rude or abusive” (offensive) flags, and how do they work? which says in part:
A post should be marked as spam only if it advertises a product, service, or similar and is unsolicited or lacks disclosure.
If the question had been barely more than a link to that target site then maybe spam, but it would have been unsuitable for our site for more usual reasons anyway.
Is it acceptable to offer money for a solution?
Probably not. Our goal is to build a library of great Q & A. It's hard to see how offering money will further that goal, and much easier to see how it might cause problems we don't want.
Ultimately this should be a community decision, but my own view is that we should remove such offers via an edit straight away.
It might be OK to allow people to progress on a paid basis after Q & A is complete here, but it seems unlikely that making a habit of that would be seen as constructive overall.
If the answer is only posted at freelancer.com, is that wrong?
Wrong from our point of view, for sure.
I think the key test is whether the question is suitable for us without the offer of payment. Edit that out (physically or mentally) then assess the remaining question on its merits.
The particular example cited seems to have been positively received after a few tweaks at this point in time.