The Text property of a Textbox control is a string which cannot be directly moved into the amount field which is a numeric.
The following should work but it assumes that the value entered is a valid numeric.
You should check the field to not being blank beforehand.
if TextBox17::Text not = ""
set amount to type System.Convert::ToDecimal(self::TextBox17::Text)
end-if
You might also look at using the maskedTextBox control instead of the standard textBox so you can define the type of data that can be entered for these numeric fields.