No TVA invoice means you don’t have to pay any VAT but you risk alienating or even losing your gardener.
Quite frankly you’re nuts to be paying a gardener on the black because both of you are breaking the law . The chèque emploi system is specifically designed for help around the house or garden and will probably cost you less than you are paying now and will remunerate your employee properly.
For example, we have someone to help with home cleaning. Last month we paid her €135 for 12hrs work and paid her social security contributions of €61. Total outlay €196. But the government will refund us €98, a full half of our total outlay, by either tax credits or cash in 2019; so go figure. We employ someone quite legally for a net cost to us of just over €8 per hour while she receives just over €11 per hour, cash or cheque in hand at the end of each month.