Funding for major defence purchase does not work like that in BD.
The government makes seperate allocations for these. It is a matter of making the case for the required hardware to the government and pushing it through the red tapes. BA and BN do it all the time.
The government cannot select fighters for BAF. BAF have to choose and insist. All they do is make half-assed attempts and buckle under the slightest pushback from the bureaucracy.
Since the current govt took office BA and BN have transformed themselves in many aspects (3 new infantry divisions, army special forces strength multiplied by 4-5 times, addition of dozens of SPHs and GMLRS, SAMs, infantry modernisation, tanks, 6-7 frigates, dozens of other vessels, upgrade and expansion of SWADS, expansion of army and navy aviation units you name it).
In the meanwhile almost everything BAF has bought is less important than procuring fighter jets.