The letter would be good news for Santos who, according to the Drug Enforcement Administration (DEA), has failed to curb coca cultivation in Colombia’s lawless rural regions.
The US president threatened earlier this year he would decertify Colombia as cooperative in international counter-narcotics efforts amid disturbing coca cultivation growth rates.
According to anonymous government officials, Trump would acknowledge the Colombian government’s efforts in the letter.
Santos would announce the letter in an address to the nation over an ongoing peace process on Friday.
Ever since the government and now-disarmed FARC guerrillas presented an agreement on how to tackle drug trafficking in 2012, poor farmers have taken to growing coca in a desperate search for public investment.
The unprecedented quantities of cocaine that are being produced threaten to undermine the peace process and decades of counter-narcotics efforts.
More than 100,000 families were expected to live off coca last year, according to the United Nations Office on Drugs and Crime.
Colombia’s front lines of the drug war
US ambassador Kevin Whitaker told media in response he was confident Colombia would reach its 50,000-hectare target of forcibly eradicated coca plants.
Eradication operations have left at least nine civilians dead this year.
Human rights organizations had warned Washington that pressure over eradication would generate more violence.
The United Nations has explicitly supported a crop substitution program combined with development in the long-neglected rural areas.
The crop substitution program, however, has barely come off the ground as the government is short of funds and drug trafficking illegal armed groups violently oppose the state in long-neglected areas.
Coca farming communities are also threatened by the illegal armed groups, creating an explosive situation.