That's strange that it won't allow you to manually enter the amount in the relevant field. Since you are using an Android smartphone, I assume that you are using the Chrome browser. To disable autofill:
1. Open Chrome browser.
2. Click on the menu icon at the top right of the taskbar (three parallel lines).
3. Click on "Settings."
4. Click on "Autofill forms."
5. Click on the switch to toggle autofill to off.