Transition car for 4 weeks as cheap as possible – or car rental

Servus, my special needs a car for 4 weeks in April, because his new car only comes later than the lease runs out from the old. He just has to go to work and back again every day about 30km. In the car rental he already asked, at Buchbinder they want to buy a cucumber for the cheapest class on the day 23€, (Fiesta, Corsa…) I thought the bookbinder would be the discounter among the car rental companies…. I think it is better to buy a cucumber here for about 700€ and this one after de 4 weeks to get a little shot, and repel again, right? Think, there could be a little win in the end, right?? Who has experiences here?? Looking forward to your info.