I think you misjudge Lowe's value. Derby could easily get a couple of mil for him from another team down south due to his age and time left on his contract. I don't think it's realistic to expect someone to buy Cosgrove from us - especially since you consider him and EFL1 level striker - at a price which would allow us to buy Lowe, who you think has the potential to be a £4m or £5m player. I don't follow your logic at all.
In answer to your hypothetical question, no I wouldn't sell Cosgrove to fund Lowe.