I'm sure a professional player nearing the end of his career will take that extra 60k per-week, and however much more the signing-on fee United is paying, to play whatever football he's asked of.
How is 29 nearing the end ? He still has a minimum of 5 years left, with 3 at a very top level. Playing in a free flowing attacking team will help his game massively and he may even get improved contract from City further down the line.