(1) A flat-rate farmer who, in accordance with section 86(1), is required to issue an invoice in respect of the supply of agricultural produce or an agricultural service shall, in respect of each such supply, issue an invoice in the form and containing such particulars (in addition to those specified in section 86(1)) as may be specified by regulations, if the following conditions are fulfilled:

(a) the issue of the invoice is requested by a purchaser;

(b) the purchaser provides the form for the purpose of the invoice and enters the appropriate particulars thereon;

(c) the purchaser gives a copy of the invoice to the flat-rate farmer,

but may issue the invoice if those conditions or any of them are not fulfilled.

(2) Where, subsequent to the issue by a flat-rate farmer of an invoice in accordance with subsection (1), the consideration as stated on the invoice is increased or reduced, or a discount is allowed, whichever of the following provisions is appropriate shall have effect:

(a) in case the consideration is increased -