I haven't received the support promised in my funding letter. What should I do?
Based on a conversation in the comments, so you have basically no reason to see this as anything but a mistake. You were never told you would be paid anything less than you were offered. You have not communicated to anyone that you are being paid less than you are supposed to be paid. Most likely no one knows you are not being paid correctly.

You have done a very silly thing by letting this go on so long. Go talk to your advisor face to face as if your advisor is a normal human being, let them know you think there is a mistake because have not been paid the amount you expected to be paid, and ask who you need to talk to in order to get this fixed or whether you have misunderstood something. If you cannot meet face to face, you can say the same thing in an email.

There is no need for this to be damaging to your relationship, there is no need for you to accuse anyone of mistreating or abusing you, there is no broken relationship already. It sounds like there is an administrative error that needs to be corrected. Administration in a university is complicated and involves people that don't know you or your advisor personally at all, they may have nothing in their records to show what you should be paid. The likelihood of mistakes increases dramatically if you are supposed to be paid from more than one source.

Only if your advisor refuses to correct the mistake and refuses to let you know who you need to talk to to correct the mistake should you assume any malice.