Read Exodus 11
God told the people to ask of the Egyptians for silver and gold jewelry, and they gladly gave it to them.
We have an advantage today of knowing what the silver and gold are to be used for but they didn't then. Some people look at this as the back wages being paid, I suppose one could make that consideration. In the wilderness two things will happen, they will have opportunity to make an idol, and they will need the precious metals to make the tabernacle accessories. When we get to the chapters concerning the tabernacle we'll look more closely at how much but suffice it to say there were tons of silver and gold.