It is natural to assume that one could pay in at any time and in any amount. But it is not that simple.
A buy-in is not possible in every case and not in unlimited amounts. The precondition is that a pension gap exists at all, that is, a difference between the retirement balance already saved and the maximum possible under the regulations. Only this gap defines how much may be paid in voluntarily. Anyone who has already reached their regulatory maximum cannot buy in further, even if they would like to. A buy-in therefore does not depend solely on whether one is in gainful employment or whether the employer agrees, but on whether there is a gap that can be filled.
This limitation makes sense, because tax-privileged occupational pension provision is meant to serve the build-up of an appropriate pension and not to act as an unlimited tax-saving vehicle. Anyone planning a buy-in should therefore first examine their personal pension situation, ideally with professional guidance, in order to use the available scope optimally and at the right moment.