Net spend shows the relative value of the quality of players added/lost from a squad. It's not that difficult to understand unless your a person you doesn't appreciate context.
It has nothing to do with credit. It's about assessing the relative value of what lost and gained in the squad due to transfers. Simple as. Would a business only analyze revenues without looking at losses?
It's really simple stuff.
You have used net spend as a tool to show that Mourinho has done a worse job because "he has spent more". This is an irrational way to use net spend when so much of what makes up the net spend is outside of the manager's control.
Sales price negotiations, the success of sales negotiations (e.g. Mourinho clearly wanted to sell Darmian, but negotiations failed), the value of the squad inherited are all outside of the manager's control, so why are you using it as a way to compare performance?
It is called responsibility accounting: manager's should only be held responsible for what they can control. How can you expect Mourinho to lower his net spend to the same as Solskjaer when the players he inherited are worth far less. Mourinho sold Depay and Schneiderlin for a combined £35m, whilst Solskjaer sold Lukaku alone for £70m. He may be doing a bad job footballing-wise, but this is something Woodward will understand.
You have also not taken into account prize money, mentioned by another poster, that has come as a result of those signings. This would also need to be netted off against the purchases as it is the main purpose of investment: winning trophies, getting higher up the table, getting in the Champion's League etc. and, as a result, getting a larger amount of prize money. £40m was gained from winning the Europa League alone, which was as a result of the investment in players.
Mourinho spent £151m gross in his first summer. He was able to sell Schneiderlin and Depay for £35m combined. Net spend is £116m
Solskjaer spent £145m gross in his first summer. He was able to sell Lukaku for £70m. Net spend is £75m.
There is very little difference between their gross figures. The only reason Solskjaer's net is so low is that he had such a valuable player to sell which he inherited from the past squad. Mourinho had no such player to sell. Essentially, you are saying that the manager's performance should be judged on events that happened before he was the manager. This is why it is nonsensical.
Net spend with prize money for Mourinho: £151m - £35m (Depay and Schneiderlin) - £30m (Sixth place league money) - £1.8m (League Cup winner) - £26m (Champions League entry for Manchester United) - £40m (Europa League Winner total) = £18m. Suddenly, his net spend is lower than Solskjaer based on his return.