Tax-free shopping: foreign tourists are eligible to reclaim tax (VAT) on purchases of domestically-manufactured goods with a value over ARS $70 when made at participating outlets that belong to Global Blue Argentina. To do so you will need to keep your receipts and complete the relevant forms at customs when leaving the country. Information on where to claim the refund can be found here: https://www.globalblue.com/tax-free-shopping/argentina/buenos-aires-port1
Keep some small change handy: shops sometimes run low on small change and will be very happy with you if you can pay with the exact amount when paying in cash.
Using leftover pesos: If you have a large amount of pesos left at the end of your visit, you can change up the equivalent of US$ 100 at an official casa de cambio (note that while most casas de cambio will stock US dollars, euros and Brazilian reales, other currencies may not be available.) You may be asked to provide receipt from your original sale of in order to buy back dollars. If you have a small amount of pesos left over, it is probably best to use it to treat yourself to some souvenirs.