All the players playing in IPL, whether Indian or foreign, are paid in Indian currency only. This is because IPL is an Indian league and runs in Indian rupees.
Apart from this the Indian Rupee is the legal currency in India. All transactions are done in this currency. Apart from this, according to the Income Tax law in India, tax has to be paid only on the income in Indian Rupees. If payments are made in foreign currency, players may suffer losses due to tax rate fluctuations. Apart from this, many other leagues around the world also have similar rules, where players are paid in the currency of the country where the league is held.
Players get this benefit
Players get many benefits when paid in Indian Rupees. Since players can only spend in Indian Rupees while in India. Apart from this, sometimes players also get some special tax related facilities and many banks in India provide banking facilities to foreign players.
But some things also depend on consent. For example if it is written in the contract, sometimes players may get a certain amount in foreign currency as per their contract and the rest may be paid in rupees.