in xml view

<field name="amount_paid" placeholder="Amount Paid" on_change="onchange_amount_paid(amount_paid)"/>

function defenition can be

def onchange_amount_paid(self, cr, uid, ids, amount_paid, context=None): res = {'value':{}}

if amount_paid:

if fee_type==1 and amount_paid<70:

warning = { 'title': ("Warning"), 'message': ('registration account minimum payment is 70'), }

return {'value': res.get('value',{}), 'warning':warning} 

return {'value': res.get('value',{})}

this will validate the amount paid in onchange function. if the amount is less than 70 it will show an error