Based on what? The 1st investor should know better. They should say, "we believe in you, and we want to offer the additional funding." This is not a game. If OP has something that investors are interested in then they should be able to raise cash for the good of all shareholders. The OP should go to the original investor and offer them the first right of refusal. This is a business. The angel should say, "cool here is the 150K" if they believe in the company. This is either a new angel that has have never done this before or an angle that is trying to take advantage of a new founder. The right thing for the OP to do is to tell the investors they need addtional funding and ask if they want to particpate.

For what it is worth I have invested in angel rounds, arranged for a startup to receive angel rounds, done a few startups and have held board seats. Also to the OP - warrants with a note is something to look at.