public class selectedTasks {
    private final List<agreement__c> agreementTasks;
    public String agreementId {set;get;}

    public agreement__c selectedAgreement {set;get;}
    private String finalQuery = '';

    public selectedTasks() {
        try{
            agreementTasks = [SELECT name FROM agreement__c WHERE Id =:agreementId ];

        } catch(Exception e){
            errorMessage = 'Id : '+agreementId;
        }
    }
}

<apex:component controller="selectedTasks" access="global">
    <apex:attribute assignTo="{!agreementId}"
                    name="agrId" type="String"
                    description="Agreement ID"
                    access="global"
                    required="true"/>
    {!errorMessage}
</apex:component>

<apex:page >
  <c:Partner_Tasks_Component agrId="a0Z25000001cSXG" />
</apex:page>