How can a Master degree lead to 300 ECTS?
I'm guessing ECTS is the European Credit Transfer and Accumulation System. From that page, a bachelor's degree is worth 180-240 ECTS credits, and the master's is worth 60-120.

It sounds as though the institution you're interested in wants a master's degree and at least 300 ECTS credits. From the linked page, it's possible that you could have a master's but still only have 240 ECTS.