He paid 25% of the new home price because 100 - 75 = <<100-75=25>>25
He paid $125,000 towards the new home because 500,000 x .25 = <<500000*.25=125000>>125,000
Therefore he paid $100,000 for his first home because 125,000 / 1.25 = <<125000/1.25=100000>>100,000
#### 100000