£20m is too expensive for a 29 year old player who in addition wants to be put on a Wayne Rooney salary bracket!!!
For such a player, you either buy cheap and pay a high salary or you buy expensive pay low salary. High transfer fee, high salary for a 29 year old player is a NO No. Add to that his injury history and the fact that Arsenal and Man United are in direct competition and the deal looks very unlikely.
I also don't see a deal happening under those terms. I'm not holding my breath on this one! Actually the £20m plus approximately £42m in salary over the next four years can be put to better use!
Never count on making a good sale. Have the purchase price be so attractive that even a mediocre sale gives good returns.