Not able to purchase outcome with 0.001 Collatearal

[pimato]

Describe the bug
It seems like the UI is throwing an error when I am trying to buy an outcome with 0.001 DAI

To Reproduce
Steps to reproduce the behavior:

1. Connect to xDai tight integration
2. Go to http://localhost:3000/#/0x617df4629b2b11ab775562b69ff20e148fe77fb8
3. Click on 'Buy - Button'
4. Set 0.001 as purchase amount
5. Click on Buy - Button
6. See error: gnosis-conditional-exchange::Market::Buy - invalid input argument (arg="investmentAmount", reason="invalid number value", value={"_hex":"-0x093cafac6a8000"}, version=4.0.46)

Expected behavior
User should be able to purchase 0.001 value worth of outcome tokens.

Screenshots

Bildschirmaufnahme.2021-08-12.um.11.18.41.mov

[Mi-Lan]

@pimato Hey men I looked at it seems that the issue is that when we subtract the relay fee from that amount it ends up being negative value...