The answer is :
The redemption of masser sheni is similar to the redemption of Hekdesh- items that one promised to give as a donation to the Temple.
Talmud Eruchin 29a Hekdesh worth any amount can be redemed on a small coin- “peruta”. In the time of the Temple, one was not supposed to do so, as the temple would loose out, but when there is no Temple, the temple is not loosing any money, so this can be done.
Maimondies Laws of Masher sheni chapter 2 law 2 states that the Geonim ruled that masher sheni is the same as hekdesh, and any amount can be redemed on a “peruta”.
The Talmud in many places brings that although the Temple is destroyed, the sanctity of Jerusalem remains- see megillah 10,zevachim 107b, therefore the mitvah still applies, just one can’t eat maser sheni nowadays, so one is obligated to redem it on a small amount of money.
I suggest you study more of the Talmud to gain an appreciation of the depth of the Halacha.
