No, there is no law governing the foreign policy of India at the moment. There is also no law or any guideline regarding diversion of national funds -to other nations. And, finally to answer your third sub-question, no, the countries cannot sue India if they do not receive the gifts (projects and other commitments) that they were promised by the concerned minister.
In India, there is separation of powers between the legislature, the judiciary and the executive. Issues related to foreign relations and policy is a matter of policy concerns in which the judiciary does not and cannot interfere. The constitution gives the Centre the power to deal with issues of foreign policy.
The binding nature of the promise depends on how the promise of the gift has been made to the other country. If only a memorandum of understanding or letter of intent ha been signed, it would not be legally enforceable. However, if a Bilateral Treaty has been entered into, it would have legal ramifications, and rights chalked out under it can be enforced. Lastly, there is no law to decide on justifiability of these foreign commitments; however the general populace of the country can always re-elect another party if they feel so strongly against promises being made by the prevalent government in the next elections.
In Re Berubari Union and Exchange of Enclaves, AIR 1960 SC 845