if phone has trade in promo, can i pay some more money on its balance but not pay it fully yet since i dont have the funds to fully pay it in one go?

Como @gramps28 mentioned.  It ultimately depends on the term of your EIP.  Customer Care at 611 or reaching out to T-Mobile via DM on Facebook or Twitter can have them take at a look at it and tell you if you can pay any extra on the device and not have that void any promotion attached to your EIP.  When I purchased my s23 Ultra, I was told up front that the only time I could make any extra payment was at the time of purchase.