It seems that ParseInt() will not work with values above $7FFFFFFF. I do know that $80000000 is greater than the maximum Int, but I would think that it would just simply roll the extra bit into the sign bit, just like this example
B4X:
Dim Int_Number As Int = 0x80000000
Log(Int_Number)
typed my reply at the same time as yours. The reason that it is negative is because the largest value that can be held by an Int is 2147483647 ($7fffffff). When you write a larger value to the Int, the extra bit is written to the sign bit, giving you a negative value. Anything larger than $FFFFFFFF will most likely be truncated to the lowest 32 bits.