We supposedly had £13m available for Negredo. According to Transfermarkt we had a net spend of £2.6m last year (Mirallas, Stones, Oviedo and Pienaar), so it can't have been the Rodwell money. If it was an advance on the tv money then there should still be about £6m available.
Martinez has made 4 signings (Kone, Alcaraz, Joel and Deulofeu) and integrated players like Oviedo, Naismith, Stones and Barkley more fully....which makes them like new signings! Eight new players to the group is quite a big change and he probably wants to see what works and what doesn't. The sensible thing would be to keep the £6m in reserve in case we have an injury problem, a bargain comes along or it later becomes apparent that an area of the team needs strengthening. Plus, if he sells players it will give him an even bigger pot to play with rather than blowing it now.