How do you know they didn't? There was lots of mis-information flying round about how much he actually had to spend. For all we know he was in full control of his budget, fully aware of the amount, and spent it accordingly.
He was desperate to sign Johnson and was probably panicked into paying way over what he expected, leaving him short elsewhere. Yes, he should have had more to spend but I'm not 100% sure he would have spent it wisely.
Fans have a very simplistic view of transfers, ignoring the reality of wages, agents fees, etc, which all come out the budget and always have.