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?

Hey there. Paying extra when you can is a great way to help make bills more manageable for longer. Kudos to you for reaching out so you're fully aware of the impact those extra payments will have so there are no surprises later. 

If you are on an EIP agreement (Not the lease), then you can always pay extra towards the device balance. When you do that, it reduces the number of months that you pay not the amount you pay each month. Nuestra Formas de pagar tu factura de T-Mobile support page can take you right to where you can make those payments on your account. Just select the One Time Payments option and then the One-Time device payments link. 

Something else to consider when making the extra payments to the device balance: when you make the payment and reduce the number of payments you make overall, it does not change the bill credit that you are getting from the promotion. Paying extra will mean that the charge for the phone will fall off of the account sooner and the bill will go down more quickly, but you should keep your same plan until the credits are finished (until the end of the 24 months) because if you change your plan, it could stop those credits. 

Hope this clarifies things for you.