Plea bargain
Plea bargain as a formal legal provision was introduced in Pakistan by the National Accountability Ordinance 1999, an anti-corruption law. A special feature of this plea bargain is that the accused applies for it, accepting guilt, and offers to return the proceeds of corruption as determined by investigators/prosecutors. After an endorsement by the Chairman National Accountability Bureau, the request is presented before the court, which decides whether it should be accepted or not. If the request for plea bargain is accepted by the court, the accused stands convicted but neither is sentenced if in trial nor undergoes any sentence previously pronounced by a lower court if in appeal. The accused is disqualified to take part in elections, hold any public office, or obtain a loan from any bank; the accused is also dismissed from service if a government official.
English contract law
Generally speaking, all parties to a contract must precisely perform their obligations or there is a breach of contract and, at the least, damages can be claimed. However, as a starting point, to claim that someone else has breached "their" side of a bargain, one must have at least "substantially performed" their own obligations. 
For example, in "Sumpter v Hedges" a builder performed £333 worth of work, but then abandoned completion of the contract. The Court of Appeal held he could not recover any money for the building left on the land, even though the buyer subsequently used the foundations to complete the job. This rule provides a powerful remedy in home construction cases to a customer. So in "Bolton v Mahadeva" Mr Bolton installed a £560 heating system in Mahadeva's house. However, it leaked and would cost £174 to correct (i.e. 31% of the price). Mahadeva did not pay at all, and the Court of Appeal held this was lawful because the performance was so defective that there could not be said to be any substantial performance. However where an obligation in a contract is "substantially performed", the full sum must be paid, only then deducting an amount to reflect the breach. So in "Hoenig v Isaacs" Denning LJ held a builder who installed a bookcase poorly, with a price of £750 but costing only £55 to correct (i.e. 7.3% of the price), had to be paid minus the cost of correction. If a contract's obligations are construed as consisting of an "entire obligation", performance of it all will be a condition precedent (a requirement before) to performance from the other side falling due, and allowing a breach of contract claim.