This is completely illogical.
Lets say a team like Arsenal come along and want to buy Bassey. They've £25m they're going to put towards the deal, on the basis of a £15m fee and 40 grand a week wages for five years.
Bassey knows this. The club know this. Arsenal know this.
So the club go to Bassey and say "We'll give you a contract extension for two years and bump your wages from £5k to £25k, this way Arsenal will have to pay more money for you".
Why does Bassey make that deal? It drives up the cost of acquiring him, no doubt - possibly as high as £20m. So Rangers benefit. But that money has to come from somewhere. And it comes from the money Arsenal are going to pay for his wages - and that's if they remain interested at all. Because if Bassey is now worth £20m, and Arsenal can now only offer similar wages to what he's on at Rangers, Bassey doesn't win out of that. There's also a high chance Arsenal decide to sack the whole deal off and look at one of the other defenders on their shortlist that costs a bit less, leaving Bassey stuck at Rangers earning a fraction of the wage he could have made.
There's no way in hell Bassey would ever do that, and that's why, you may have noticed, that kind of deal never happens.