re: "does this mean that I will not receive the full payment? the price of the milestone that has been paid is much lower than that of the contract"
You will not receive full payment.
The "price of the contract" does not matter.
The "budget" does not matter.
Promises don't matter.
What matters is money funded in escrow. That is the only money you can get.
If the client wants to send you more money using the client-side Send Bonus tool, then he may do so. But that is his choice. That is not something that Upwork gets involved with.
Danilo D wrote:
it was my first job, I accepted a contract, I thought that upwork would protect me, now nothing can be done?
Before you accept another contract, read upwork Tos so that you KNOW how they will protect you.