Erchin 30a - Who can redeem at a prorated rate?

The mishna has various restrictions regarding the rules of redeeming. For example, one cannot sell another property or borrow to obtain capital with which to redeem. Also, one is not allowed to redeem just a piece of the property, they need to wait until they have enough to redeem the entire amount. There is also a system of calculating the redemption price by dividing the sale price by the number of years (with a few more details to compensate for fluctuations in the price of the property, and if it is resold at a different price), from hereon in referred to as prorating. The Rashash questions whether these details of redemption apply specifically to the owner of the property or even to his relatives when they redeem in his place. The Rashash quotes from Toras Kohanim that all the rules apply to the relatives also, except for the concept of prorating which only applies to the actual owner. The Rashash says that the pesukim clearly imply "to one who has a sharp eye" that only for an owner do we prorate to force the buyer to sell him his field back, but when a relative redeems we don't impose this on the original buyer, rather we make the relative redeem at full price.

The Torah talks about prorating both in the context of an inherited field (Parshas Behar 25:25-27) and when one is sold to a ger toshav (48-50). In both places the torah first speaks about relatives redeeming for him but then says that if he comes across money so that he can redeem it by himself or redeem himself, the price is prorated. From this the Rashash understands that it is only prorated when he redeems, not when his relatives redeems.

However, the Meshech Chochma seems to take for granted that even when relatives redeem it is prorated. He asks why does the Torah speak first about relatives redeeming and then about redeeming himself. If relatives can redeem him, surely he can redeem himself when he gets the money together. The Meshech Chochma suggests based on chazal who understand from the pesukim that the prorating must be done meticulously so that the buyer is not being cheated in any way. Therefore, it first says that when relatives redeem we have to be careful to make sure the buyer isn't cheated. Even if the seller himself redeems, he is required to be meticulous about the calculation and not cheat the buyer. The Meshech Chochma seems to understand that both the redemption of the seller and the relatives are included in the system of prorating (unlike the rashash) but it is more of a chiddush to say that even the seller himself must be meticulous about the calculation.