There is always liability and asking or paying your tenant to use a power tool that could hurt them increases the liability. If the blade falls off and they chop off their toe, the situation becomes an opportunity for a lawyer to look into what kind of assets you might be willing to part with. You would be surprised how many seemingly capable people can hurt themselves by accident. I have it in my contracts state that the tenant is responsible for yard maintenance and it must be maintained to HOA standards (HOA exists in my case) and provide no further details. There is liability here also, but I didn't hand a potentially dangerous tool to a tenant. They could simply hire a service or obtain their own tool if they choose. So, I'd not offer any of your tools.
You are trying to be a nice guy and offer the tenant a discount and even offer the tools to help out. In another country where common sense is more valued, this would be great. Unfortunately the litigation in this country would prevent me from offering this solution.
If you do move forward with this, I suggest you leave the lease untouched and let him know either verbally or on a separate note that you will offer $30 credit for the months you need the lawn mowed, payable on completion of the work.