3,532. Neglecting to Pay the Extra Fifth

Hilchos Maaser Sheini and Neta Revai 5:12

One who redeems second tithe for himself, paying the principal but not paying the extra fifth, may not eat that produce until he pays the fifth; this is the case even on Shabbos. This is so even though failure to pay the fifth doesn’t impede the transfer of the sanctity; the sanctity is effectively transferred. The Sages enacted this rule out of concern that one might be careless and fail to pay the extra fifth.

Hilchos Maaser Sheini and Neta Revai 5:13

If someone wants to exchange money from redeeming second tithe for gold dinars in order to lighten his load, he is permitted to do so. One need not add a fifth in such a case, even if he made the exchange using his own money, because this is not considered redeeming it.