check sales credit note cannot be greater than outstanding balance

[shoulders]

just check for the validations, i think I have already done this. it probably checks against balance

[shoulders]

this works but

• vouchers are not copied to sales credit note but should be in reference to should i create a 'credit invoice' option #1976
• 'add sales credit note' button is currently doing 'credit invoice feature' should i create a 'credit invoice' option #1976